Solid State Lighting (SSL) technology has recently been increasingly used in various fields of lighting, such as general lighting, entertainment and automotive lighting.
The latter applications may be generally divided into two broad categories: exterior lighting (outer front and rear lamps of the vehicle) and interior lighting (interior ambient, reading and instrument cluster lighting).
One or more embodiments may mainly refer to the possible application in the automotive field, e.g. in lighting devices adapted to be used for the so-called “retrofit” in vehicle headlamps.
International regulations concerning vehicle headlamps define for example that, e.g. for a front headlamp application, the following functions may be included: high and low beam, Daytime Running Light (DRL), front position, turn indicator and front fog lamps.
In order to be homologated and installed in a vehicle, each function must achieve certain photometrical values as defined in the regulations. This means, for example, that a lamp may be required to generate a light beam which is shaped so that the luminous intensity falls within a range of minimum and maximum values in some angular points.
For example, the functions of high and low beam or the fog lamp function may require a higher luminous intensity than other functions, and therefore may require sources with high flux.
For such applications so-called H-type lamps or bulbs may be used, the most common types belonging to the categories H7, H8, H10, H11 and H16, as defined by UNECE Regulations.
In a conventional arrangement, the optical system may comprise an incandescent light source that generates the light radiation, a reflector adapted to collect light radiation in order to project it forwards and a lens.
The optical system may be designed while taking into account the geometric features of the lamp or bulb, such as the position and the size of the filament, the emission pattern of the light coming from the bulb and the total luminous flux emitted.
Various efforts have recently focused on the production of H-type bulbs by resorting to a LED technology, which may be used to replace the traditional incandescent bulbs.
The most challenging task is probably the development of a LED device adapted to replace an incandescent lamp of the front headlamps, while complying with the photometrical requirements provided by the regulations, i.e. a LED device having a light emitting volume, a radiation pattern and a total flux which are similar to an incandescent device.
In this respect, a factor which must be taken into account is given by the difference of the light emission in an incandescent filament and in a LED.
An incandescent filament emits the light radiation in a substantially anisotropic pattern around the filament axis.
On the contrary, a LED emits light from a solid-state chip towards a half-space (hemisphere) according to a pattern which may be a lambertian pattern.
A possible solution is the symmetrical arrangement of the LEDs around what may be considered as the axis of a traditional filament.
This solution has however various drawbacks in its application.
For example, the emitting volume may be definitely higher than the emitting volume of the filament. This may lead to having a light emission in areas which are out of the focus of the reflector: in applications such as high/low lamps, it may then be difficult to meet certain requirements due to the need of avoiding glaring above a certain horizontal line.
WO 2006/054199 A1 describes a light guide coupled to an SSL source, for driving the light towards an out-coupling structure. The size and position of the out-coupling structure may be chosen so as to be similar to the size and position of the filament of a traditional bulb. This out-coupling structure may include a rough surface, cuts or notches on the surface of a glass fibre.
JP 2011/023299 A shows a LED facing an optical system adapted to diffuse light. The optical system may be refractive, and some surfaces may deviate the direction of the light rays by employing reflective surfaces.
WO 2013/071972 A1 regards a solution wherein LED light radiation sources are arranged in the area which is supposed to host the filament of a traditional bulb, but without resorting to refractive or reflective optical systems.
Despite the intensive development activity, the evidence whereof is provided by the above documents, the need is still felt of solutions adapted to overcome the previously outlined drawbacks.